Nonaqueous electrolyte secondary battery

ABSTRACT

A non-aqueous electrolyte secondary cell that has excellent high-temperature cycle characteristics and that is highly safe enough to prevent overcharge is provided. The non-aqueous electrolyte secondary cell has a positive electrode for reversibly intercalating-deintercalating lithium ions, a negative electrode for reversibly intercalating-deintercalating lithium ions, and a non-aqueous electrolyte having a non-aqueous solvent and an electrolyte salt. The non-aqueous solvent includes a cycloalkylbenzene derivative and an alkylbenzene derivative having a quaternary carbon directly bonded to a benzene ring and not having a cycloalkyl group directly bonded to the benzene ring.

TECHNICAL FIELD

The present invention relates to an improvement of non-aqueouselectrolyte secondary cells having a positive electrode for reversiblyintercalating-deintercalating lithium ions, a negative electrode forreversibly intercalating-deintercalating lithium ions, and a non-aqueouselectrolyte.

BACKGROUND ART

In recent years, there has been a rapid reduction in the size and weightof mobile information terminals such as mobile telephones, notebookpersonal computers, and PDA. Higher capacity is required of cells andbatteries serving as the driving power sources of such terminals.Non-aqueous electrolyte secondary cells represented by lithium ionsecondary cells have high energy density and high capacity and as suchare widely used as the driving power sources of the mobile informationterminals.

Generally, non-aqueous electrolyte secondary cells use a positiveelectrode made of a lithium-containing transition metal compound oxide,a negative electrode made of carbon material such as graphite, and anon-aqueous electrolyte containing a lithium salt dissolved in anon-aqueous solvent. In such cells, there is migration of lithium ionsbetween the positive electrode and the negative electrode during chargeand discharge, and internal short circuiting caused by dendrite lithiumdoes not occur because no lithium exists in the state of metal. Suchcells therefore excel in safety.

However, such non-aqueous electrolyte secondary cells can be problematicin that overcharge causes an excessive release of lithium ions from thepositive electrode and an excessive storage of the lithium ions in thenegative electrode. This lowers the thermal stability of bothelectrodes, deteriorating cell characteristics. Also, an extremelyunbalanced differential between the electrodes decomposes theelectrolytic solution. The decomposition of the electrolyte causes, aswell as gas generation, heat generation resulting from an increase ininternal cell resistance. As a result, there can be a rapid increase ininternal cell pressure, causing cell burst and thermal runaway.

In view of these problems, the non-aqueous electrolyte secondary cellshave incorporated therein a current-cutting device for cutting anovercharged current, upon generation of such a current. However, thecurrent-cutting device operates to cut the current only upon increase ininternal cell pressure, and there is a time-lag between abnormality tooccur in the cell and the increase of internal cell pressure. Thus, ittakes a long time before the current-cutting device operates, and thereis a doubt as to ensuring security in the case of an intense temperatureincrease.

Also in view of the problems, there have been proposed techniques ofadding various additives in the non-aqueous electrolyte. For instance,Japanese Unexamined Patent Publication No. H5-36439 discloses atechnique of adding a linear alkylbenzene derivative in a non-aqueoussolvent in a non-aqueous electrolyte secondary cell having acurrent-cutting device. With this technique, at the time of overcharge,the linear alkylbenzene derivative is dissolved to generate methanesthat in turn consume an active oxygen detached from the positiveelectrode by reacting with the oxygen. Thus, this technique is aimed atpreventing a temperature increase caused by the active oxygen. However,since the linear alkylbenzene derivative operates neither to cut anovercharged current nor to increase the response rate of thecurrent-cutting device, the technique cannot ensure security in the caseof an intense temperature increase.

Japanese Unexamined Patent Publication No. H9-106835 discloses atechnique of preventing overcharge by using a non-aqueous solvent havingadded therein thiophene, biphenyl, furan, and the like. With thistechnique, the compounds thiophene, biphenyl, and furan polymerize at apotential higher than or equal to the highest cell operation voltage andform a highly resistive film on the electrode surfaces, thus trying toprevent overcharge. However, this technique is problematic in that theabove compounds cause to lower power generation performance, and thatsince the compounds polymerize only under a high temperature of 120° C.or higher the current cutting-off cannot be realized with the use of thecompounds unless the cell temperature becomes high.

On the other hand, the present inventors suggested in JapaneseUnexamined Patent Publication No. 2001-15155 a technique of preventingovercharge by adding in a non-aqueous solvent a cycloalkylbenzenederivative or an alkylbenzene derivative having a tertiary carbonadjoining a phenyl group. These additives, suggested by the presentinventors, are chemically decomposed at the time of overcharge andgenerate a hydrogen gas, and the molecules polymerize together to form afilm on the negative electrode surface. This film is stable andinsoluble in the non-aqueous solvent and has high electrical resistance.With this technique, the hydrogen gas generated from the electrode andthe highly resistive film operate to rapidly increase internalresistance and prevent overcharge, thereby ensuring security in the caseof an intense temperature increase.

When the non-aqueous solvent having added therein the above additives isused in a non-aqueous electrolyte secondary cell provided with thecurrent-cutting device, the following advantageous effect is obtained inaddition to the above effect. The hydrogen gas generated from theelectrode increases internal cell pressure, thereby increasing theresponse rate of the current-cutting device. It should be noted,however, that these compounds are decomposed to form a highly resistivefilm on the active material surface of the negative electrode if thecell is used under a high temperature environment of 40° C. to 60° C.,regardless of the overcharged state. Thus, when using the cell under ahigh temperature environment, there is the problem of lowering cellperformance such as cell cycle characteristics.

DISCLOSURE OF THE INVENTION

In view of the foregoing and other problems, it is an object of thepresent invention to provide a non-aqueous electrolyte secondary cellthat has good cycle characteristics and that is highly safe enough toprevent overcharge. It is another object of the present invention toprovide, where the current-cutting device is provided, a highly safenon-aqueous electrolyte secondary cell that increases the responsivenessof the current-cutting device.

These objects of the present invention can be accomplished by thefollowing structures.

(A) A non-aqueous electrolyte secondary cell comprising a positiveelectrode for reversibly intercalating-deintercalating lithium ions, anegative electrode for reversibly intercalating-deintercalating lithiumions, and a non-aqueous electrolyte having a non-aqueous solvent and anelectrolyte salt, wherein the non-aqueous solvent includes acycloalkylbenzene derivative and an alkylbenzene having a quaternarycarbon directly bonded to a benzene ring and not having a cycloalkylgroup directly bonded to the benzene ring.

In the cycloalkylbenzene derivative, a hydrogen atom bonded to α carbon(a carbon directly bonded to the benzene ring) in the cycloalkyl grouphas high reactivity, and as such this hydrogen is easy to be pulled outat the time of overcharge. Thus, at the time of overcharge thecycloalkylbenzene derivative is rapidly decomposed at the negativeelectrode to generate a hydrogen gas, and the cycloalkylbenzenederivative itself polymerizes to form a stable film on the negativeelectrode surface. This film has high electrical resistance. With thisstructure, since the generated hydrogen gas and the highly resistivefilm rapidly increase internal resistance, overcharge is restrictedbefore an intense temperature increase. In a cell provided with acurrent-cutting device that is configured to operate upon increase ininternal cell pressure, the hydrogen gas generated by the decompositionof the cycloalkylbenzene derivative increases the internal cellpressure, thereby significantly increasing the reactivity of thecurrent-cutting device.

Further, the alkylbenzene derivative having a quaternary carbon directlybonded to the benzene ring and not having a cycloalkyl group directlybonded to the benzene ring (where necessary hereinafter referred to asan alkylbenzene derivative having a quaternary carbon directly bonded tothe benzene ring) is adsorbed on the negative electrode surface to forma film so that the cycloalkylbenzene derivative would not come in directcontact with the negative electrode. This restricts the decomposition ofthe cycloalkylbenzene derivative under a high temperature, therebypreventing the deterioration of high temperature cycle characteristics.

With the above described structure, a non-aqueous electrolyte secondarycell excellent in safety is realized without deteriorating hightemperature cycle characteristics.

(2) In the structure of (1) above, the non-aqueous solvent may furtherinclude an unsaturated cyclic carbonate derivative.

This structure is more preferable in that the unsaturated cycliccarbonate derivative operates to restrict the decomposition of thecycloalkylbenzene derivative under a high temperature. This is becausethe alkylbenzene derivative having a quaternary carbon directly bondedto the benzene ring is adsorbed mainly on the basal surface of thecarbon and the unsaturated cyclic carbonate is adsorbed on a negativeelectrode portion (mainly on the edge surface of the carbon) other thanthe basal surface. That is, the alkylbenzene derivative and theunsaturated cyclic carbonate cooperate in more effectively restrictingthe decomposition of the cycloalkylbenzene derivative.

It should be noted that an alkylbenzene derivative that has both thecycloalkyl group and the quaternary carbon directly bonded to thebenzene ring mainly functions as a cycloalkylbenzene derivative,although the reason therefor is unknown. For this reason, this compoundwill be treated as a kind of the cycloalkylbenzene derivative.

(3) In the structure of (1) or (2) above, the cycloalkylbenzenederivative may be contained in the non-aqueous solvent at a ratio of 0.5to 5 parts by mass per 100 parts by mass of the non-aqueous solvent, andthe alkylbenzene derivative having a quaternary carbon directly bondedto a benzene ring and not having a cycloalkyl group directly bonded tothe benzene ring may be contained in the non-aqueous solvent at a ratioof 0.5 to 10 parts by mass per 100 parts by mass of the non-aqueoussolvent.

If the cycloalkylbenzene derivative is contained in the non-aqueoussolvent at a ratio of less than 0.5 part by mass per 100 parts by massof the non-aqueous solvent, the effect of preventing overcharge isinsufficient. If the cycloalkylbenzene derivative is contained at aratio of more than 5 parts by mass, the resistance increases because ofthe film formed on the negative electrode surface. In view of this, itis preferable that the cycloalkylbenzene derivative be contained in thenon-aqueous solvent at a ratio of 0.5 to 5 parts by mass per 100 partsby mass of the non-aqueous solvent.

If the alkylbenzene derivative having a quaternary carbon directlybonded to the benzene ring is contained in the non-aqueous solvent at aratio of less than 0.5 part by mass per 100 parts by mass of thenon-aqueous solvent, the film adsorbed and formed on the negativeelectrode surface becomes coarse, failing to obtain sufficienthigh-temperature cycle characteristics. If the alkylbenzene derivativehaving a quaternary carbon directly bonded to the benzene ring iscontained at a ratio of more than 10 parts by mass, the film adsorbedand formed on the negative electrode surface becomes dense, therebyexcessively increasing electrical resistance. In view of this, it ispreferable that this compound be contained in the non-aqueous solvent ata ratio of 0.5 to 10 parts by mass per 100 parts by mass of thenon-aqueous solvent.

If the unsaturated cyclic carbonate derivative is contained in thenon-aqueous solvent at a ratio of less than 0.5 part by mass per 100parts by mass of the non-aqueous solvent, the film adsorbed and formedon the negative electrode surface becomes coarse, failing to obtainsufficient high-temperature cycle characteristics. If the unsaturatedcyclic carbonate derivative is contained at a ratio of more than 10parts by mass, the film adsorbed and formed on the negative electrodesurface becomes dense, thereby excessively increasing electricalresistance. In view of this, it is preferable that the unsaturatedcyclic carbonate derivative be contained in the non-aqueous solvent at aratio of 0.5 to 10 parts by mass per 100 parts by mass of thenon-aqueous solvent.

While the cycloalkylbenzene derivative is not to be particularlyspecified, for instance, cyclopentylbenzene and cyclohexylbenzene can beconveniently used.

While the alkylbenzene derivative having a quaternary carbon directlybonded to the benzene ring is not to be particularly specified, forinstance, tert-butylbenzene, tert-amylbenzene, and tert-hexylbenzene canbe conveniently used.

While the unsaturated cyclic carbonate derivative is not to beparticularly specified, for instance, a compound having the structureshown as Chemical Formula 1 below can be conveniently used.

where R1 and R2 are independently a hydrogen atom or an alkyl group withsix carbon atoms or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a non-aqueous electrolytesecondary cell according to the present invention.

FIG. 2 is an enlarged half-sectional view of a sealing structure of thenon-aqueous electrolyte secondary cell according to the presentinvention.

In the figures, reference numeral 1 refers to a positive electrode,reference numeral 2 to a negative electrode, 3 to a separator, 4 to anelectrode assembly, 5 to an outer casing can, 6 to a sealing structure,7 to a terminal cap, 8 to an explosion-proof valve, 9 to a sealingplate, 10 to a positive current collector tab, 11 to a negative currentcollector tab, 12 to a PCT element, 13 to a gasket, 13 a to an outergasket, 13 b to an inner gasket, 14 and 15 to insulation plates, 16 to acaulking margin, 17 and 18 to gas releasing holes, 19 to inner portionof the sealing structure, and 20 to a cell body portion.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described with referenceto the drawings. It should be noted that the present invention is notlimited to the following embodiment; it will be appreciated thatvariations are possible without departing from the scope of theinvention.

FIG. 1 is an exploded perspective view of a non-aqueous electrolytesecondary cell according to the embodiment of the present invention.FIG. 2 is an enlarged half-sectional view of a current-cutting sealingstructure provided at the opening portion of the outer casing can shownin FIG. 1.

Referring to FIG. 1, a lithium ion cell according to an example of thepresent invention has a cylindrical outer casing can 5 provided with abottom. In the outer casing can 5 is encased a spiral-shaped electrodeassembly 4 formed of a positive electrode 1 in which an active materiallayer mainly made of LiCoO₂ is formed on a substrate made of aluminum, anegative electrode 2 in which an active material layer mainly made ofgraphite is formed on a substrate made of copper, and a separator 3 thatseparates the electrodes 1 and 2. Further in the outer casing can 5 isinjected an electrolytic solution in which LiPF₆ is dissolved at a ratioof 1M (mole/liter) in a mixture solvent in which ethylene carbonate (EC)and diethyl carbonate (DEC) are mixed at a mass ratio of 4:6. To theopening portion of the outer casing can 5, a sealing structure 6 iscaulked and fixed with the intervention of an insulating outer gasket 13a made of polypropylene (PP). Thus, the cell is sealed with the sealingstructure 6.

The sealing structure 6, as shown in FIG. 2, has a sealing plate 9 madeof an aluminum alloy. To the sealing plate 9, an explosion-proof valve 8that has an approximately hemisphere center portion and is made of analuminum alloy, a PTC element 12, and a terminal cap 7 provided with agas releasing hole 18 are caulked and fixed with the intervention of aninsulating inner gasket 13 b made of polypropylene (PP). Theexplosion-proof valve 8 delimits an inner portion 19 of the sealingstructure and a cell body portion 20 (the portion in which the electrodeassembly 4 is encased). In a normal state, the explosion-proof valve 8is electrically connected to the sealing plate 9. When abnormality suchas overcharge occurs and internal cell pressure exceeds a predeterminedvalue, the valve 8 is separated from the sealing plate 9 because of theinternal cell pressure, thereby discontinuing the charge.

To the outer casing can 5 is connected a negative current collector tab11 electrically connected to the negative electrode 2, and to thesealing plate 9 of the sealing structure 6 is connected a positivecurrent collector tab 10. Thus, chemical energy generated inside thecell is turned into electric energy that is brought outside. Near bothtop and bottom end portions of the electrode assembly 4 are disposedinsulation plates 14 and 15 for preventing short circuiting inside thecell.

In the above electrolyte solution are added a cycloalkylbenzenederivative and an alkylbenzene derivative having a quaternary carbondirectly bonded to the benzene ring and not having a cycloalkyl groupdirectly bonded to the benzene ring.

The negative electrode material used herein can be natural graphite,carbon black, coke, glass carbon, carbon fiber, or a carbonaceoussubstance such as a baked body of the foregoing, or a mixture of thecarbonaceous substance and one or more substances selected from thegroup consisting of lithium, a lithium alloy, and a metal oxide capableof reversibly intercalating-deintercalating lithium.

The positive electrode material used herein can be one oflithium-containing transition metal compound oxides, or a mixture of twoor more of the foregoing. Examples include lithium cobalt oxide, lithiumnickel oxide, lithium manganese oxide, lithium metal oxide, or an oxidein which a part of the transition metal contained in the above oxides issubstituted by another element.

The non-aqueous solvent used for the electrolytic solution can be amixture of a high-permittivity solvent in which a lithium salt is highlysoluble and a low-viscose solvent. Examples of the high-permittivitysolvent include ethylene carbonate, propylene carbonate, butylenecarbonate, and γ-butyrolactone. Examples of the low-viscose solventinclude diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate,1,2-dimethoxy ethane, tetrahydrofuran, anisole, 1,4-dioxane,4-methyl-2-pentanone, cyclohexanone, acetonitrile, propionitrile,dimethylformamide, sulfolane, methyl formate, ethyl formate, methylacetate, ethyl acetate, propyl acetate, and ethyl propionate. Thehigh-permittivity solvent and the low-viscose solvent each may be amixture solvent of two or more of the foregoing.

The electrolyte salt used herein can be one compound or a mixture of twoor more compounds selected from the group consisting of LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)₂, LiClO₄, LiPF₆, and LiBF₄. The amount of these electrolytesalts dissolved in the non-aqueous solvent is preferably 0.5 to 2.0mole/liter.

The material of the sealing plate 9 is not limited to the above aluminumalloy; it is also possible to use metal aluminum, iron, stainless steel,or the like.

The present invention will be further detailed with the use of Examples.

EXAMPLE 1

A non-aqueous electrolyte secondary cell according to Example 1 wasprepared in the following manner.

Ninety two parts by mass of a positive electrode active material made oflithium cobalt oxide (LiCoO₂), 5 parts by mass of a carbon-basedconductivity agent made of acetylene black, 3 parts by mass of a bindermade of polyvinylidene fluoride (PVdF), and N-methyl-2-pyrrolidone (NMP)were mixed, thus obtaining an active material slurry.

This active material slurry was uniformly applied on both surfaces of apositive electrode substrate made of an aluminum foil of 20 μm thick bydoctor blade, and this resulting article was dried while passed througha heated dryer. By this drying step, an organic solvent required in thestep of preparing the slurry was removed. Subsequently, this electrodeplate was rolled with a roll press machine so that the thickness of theelectrode plate was made to 0.17 mm, and thus, the positive electrode 1was prepared.

Ninety five parts by mass of a negative electrode material made ofgraphite, 5 parts by mass of a binder made of polyvinylidene fluoride(PVdF), and N-methyl-2-pyrrolidone (NMP) were mixed, thus obtaining anactive material slurry. This material slurry was uniformly applied onboth surfaces of a negative electrode substrate made of a copper foil of20 μm thick by doctor blade, and this resulting article was dried whilepassed through a heated dryer. By this drying step, an organic solventrequired in the step of preparing the slurry was removed. Subsequently,this electrode plate was rolled with a roll press machine so that thethickness of the electrode plate was made to 0.14 mm, and thus, thenegative electrode 2 was prepared.

Forty parts by mass of ethylene carbonate (EC) and 60 parts by mass ofdiethyl carbonate (DEC) were mixed to prepare a mixture solvent, andLiPF₆ for serving as the electrolyte salt was dissolved in the mixturesolvent at a concentration of 1M (mole/liter). In this mixture solventwere added 1 part by mass of cyclohexylbenzene (CHB) and 2 parts by massof tert-amylbenzene (t-AB). Thus, the electrolytic solution wasprepared.

Next, the electrode assembly 4 was prepared by winding the positiveelectrode 1 and the negative electrode 2 with the separator 3 (25 μmthick) made of a microporous film of polyethylene interposedtherebetween. Then, the electrode assembly 4 was enclosed in the outercasing can 5 together with the insulation plate 14, and the negativecurrent collector tab 11 was welded to the bottom of the outer casingcan 5.

Then, the explosion-proof valve 8, the PTC element 12, and the terminalcap 7 were caulked and fixed to the sealing plate 9 with theintervention of the inner gasket 13 b. Thus, the inner portion 20 of thesealing structure was sealed. Then, the positive current collector tab10 was welded to the sealing plate 6, and the electrolytic solutionprepared above was injected into the outer casing can 5. Then, thesealing plate 6 was caulked and fixed to the opening end portion of theouter casing can 5 with the intervention of the outer gasket 13 a. Thus,a cell A1 of the present invention according to Example 1 was prepared.The nominal capacity of the cell thus prepared was 1500 mAh.

EXAMPLE 2

A cell A2 of the present invention according to Example 2 was preparedin the same manner as Example 1 except that 0.5 part by mass of thecyclohexylbenzene was added.

EXAMPLE 3

A cell A3 of the present invention according to Example 3 was preparedin the same manner as Example 1 except that 5 parts by mass of thecyclohexylbenzene was added.

EXAMPLE 4

A cell A4 of the present invention according to Example 4 was preparedin the same manner as Example 1 except that 6 parts by mass of thecyclohexylbenzene was added.

EXAMPLE 5

A cell A5 of the present invention according to Example 5 was preparedin the same manner as Example 1 except that 0.5 part by mass of thetert-amylbenzene was added.

EXAMPLE 6

A cell A6 of the present invention according to Example 6 was preparedin the same manner as Example 1 except that 10 parts by mass of thetert-amylbenzene was added.

EXAMPLE 7

A cell A7 of the present invention according to Example 7 was preparedin the same manner as Example 1 except that 12 parts by mass of thetert-amylbenzene was added.

EXAMPLE 8

A cell A8 of the present invention according to Example 8 was preparedin the same manner as Example 1 except that vinylene carbonate (VC) wasadded.

EXAMPLE 9

A cell A9 of the present invention according to Example 9 was preparedin the same manner as Example 8 except that 0.5 part by mass of thecyclohexylbenzene was added.

EXAMPLE 10

A cell A10 of the present invention according to Example 10 was preparedin the same manner as Example 8 except that 5 parts by mass of thecyclohexylbenzene was added.

EXAMPLE 11

A cell A11 of the present invention according to Example 11 was preparedin the same manner as Example 8 except that 6 parts by mass of thecyclohexylbenzene was added.

EXAMPLE 12

A cell A12 of the present invention according to Example 12 was preparedin the same manner as Example 8 except that 0.5 part by mass of thetert-amylbenzene was added.

EXAMPLE 13

A cell A13 of the present invention according to Example 13 was preparedin the same manner as Example 8 except that 10 parts by mass of thetert-amylbenzene was added.

EXAMPLE 14

A cell A14 of the present invention according to Example 14 was preparedin the same manner as Example 8 except that 11 parts by mass of thetert-amylbenzene was added.

COMPARATIVE EXAMPLE 1

A comparison cell X1 according to Comparative Example 1 was prepared inthe same manner as Example 1 except that no additives were added.

COMPARATIVE EXAMPLE 2

A comparison cell X2 according to Comparative Example 2 was prepared inthe same manner as Example 1 except that the tert-amylbenzene was notadded.

COMPARATIVE EXAMPLE 3

A comparison cell X3 according to Comparative Example 3 was prepared inthe same manner as Example 1 except that the tert-amylbenzene was notadded and 2 parts by mass of the cyclohexylbenzene was added.

COMPARATIVE EXAMPLE 4

A comparison cell X4 according to Comparative Example 4 was prepared inthe same manner as Example 1 except that the cyclohexylbenzene was notadded.

COMPARATIVE EXAMPLE 5

A comparison cell X5 according to Comparative Example 5 was prepared inthe same manner as Example 1 except that the cyclohexylbenzene was notadded and 5 parts by mass of the tert-amylbenzene was added.

COMPARATIVE EXAMPLE 6

A comparison cell X6 according to Comparative Example 6 was prepared inthe same manner as Example 1 except that the cyclohexylbenzene andtert-amylbenzene were not added and 2 parts by mass of cumene was added.

COMPARATIVE EXAMPLE 7

A comparison cell X7 according to Comparative Example 7 was prepared inthe same manner as Example 1 except that the cyclohexylbenzene andtert-amylbenzene were not added and 2 parts by mass of trimellitic esterwas added.

COMPARATIVE EXAMPLE 8

A comparison cell X8 according to Comparative Example 8 was prepared inthe same manner as Example 1 except that the cyclohexylbenzene andtert-amylbenzene were not added and 1 part by mass of vinylene carbonatewas added.

<Cell Characteristics Tests>

With respect to the cells A1 to A14 of the present invention and to thecomparison cells X1 to X8, overcharge tests and high temperature cycletests were conducted under the following conditions.

<Overcharge Tests>

Each cell was charged under the conditions of 1C (1500 mA) and 4.2V forthree hours and then overcharged with a charge current of 2C (3000 mA).The time between the starting of overcharge and the operation of thecurrent-cutting sealing structure was measured, and the temperatureoutside the cell was measured.

<High Temperature Cycle Tests>

Charge conditions: constant current, 1 C(1500 mA); constant voltage, 4.2V; discharge-ending current, 30 mA; and temperature, 60° C.

Discharge conditions: constant current, 1 C(1500 mA); discharge-endingvoltage, 2.75 V; and temperature, 60° C.

High temperature cycle characteristics capacity maintenance rate (%):(300-cycle discharge capacity/1-cycle discharge capacity)×100

Table 1 below lists the additives added, the amounts thereof, the timebetween the starting of overcharge and the operation of thecurrent-cutting sealing structure, the highest temperature outside thecell, and the results of the high temperature cycle tests.

TABLE 1 High temperature cycle Current Highest character- cuttingtemperature istics Cell Additives time (min) (° C.) (%) A1 1% CHB, 2%t-AB 15 74 60 A2 0.5% CHB, 2% t-AB 19 81 63 A3 5% CHB, 2% t-AB 12 70 58A4 6% CHB, 2% t-AB 12 73 50 A5 1% CHB, 0.5% t-AB 13 75 60 A6 1% CHB, 10%t-AB 14 71 61 A7 1% CHB, 12% t-AB 12 72 53 A8 1% CHB, 2% t-AB, 14 73 821% VC A9 0.5% CHB, 2% t-AB, 18 82 84 1% VC A10 5% CHB, 2% t-AB, 13 72 801% VC A11 6% CHB, 2% t-AB, 12 72 70 1% VC A12 1% CHB, 0.5% t-AB, 14 7582 1% VC A13 1% CHB, 10% t-AB, 13 72 81 1% VC A14 1% CHB, 11% t-AB, 1372 72 1% VC X1 No Additives 35 Abnormality 62 X2 1% CHB 27 Abnormality54 X3 2% CHB 14 73 45 X4 2% t-AB 35 Abnormality 61 X5 5% t-AB 35Abnormality 60 X6 2% cumene 18 83 36 X7 2% trimellitic 20 85 21 ester X81% VC 36 Abnormality 77 CHB: cyclohexylbenzene (cycloalkylbenzenederivative) t-AB: tert-amylbenzene (alkylbenzene derivative having aquaternary carbon directly bonded to the benzene ring) VC: vinylenecarbonate (unsaturated cyclic carbonate)

Where the cell temperature arose and smoking occurred, such a case isindicated as abnormality in Table 1.

Table 1 shows that the inventive cells A1 to A14, in which thecycloalkylbenzene derivative (CHB) and the alkylbenzene derivativehaving a quaternary carbon directly bonded to the benzene ring (t-AB)are added, realize high temperature cycle characteristics of 50% orhigher and the effect of preventing overcharge (a current cutting timeof 19 minutes or less and a highest temperature of 82° C. or lower).

This can be explained as follows. In the cycloalkylbenzene derivative, ahydrogen atom bonded to α carbon (a carbon directly bonded to thebenzene ring) in the cycloalkyl group has high reactivity, and as suchthis hydrogen is easy to be pulled out at the time of overcharge. Thus,at the time of overcharge the cycloalkylbenzene derivative is rapidlydecomposed at the negative electrode to generate a hydrogen gas, and thecycloalkylbenzene derivative itself polymerizes to form a stable film onthe negative electrode surface. This film has high electrical resistanceand thus restricts overcharge. In a cell provided with a current-cuttingdevice, the hydrogen gas generated by the decomposition of thecycloalkylbenzene derivative increases internal cell pressure, therebyincreasing the reactivity of the current-cutting device. Further, thealkylbenzene derivative having a quaternary carbon directly bonded tothe benzene ring is adsorbed on the negative electrode surface to form afilm so that the cycloalkylbenzene derivative would not come in directcontact with the negative electrode. This restricts the decomposition ofthe cycloalkylbenzene derivative under a high temperature, therebypreventing the deterioration of high temperature cycle characteristics.As a result, good cycle characteristics and the effect of preventingovercharge are obtained.

The cell A8, in which the unsaturated cyclic carbonate (VC) is added, issuperior in high temperature cycle characteristics to the cell A1without VC added therein. This is because the unsaturated cycliccarbonate has an adsorption portion different from that of thealkylbenzene derivative having a quaternary carbon directly bonded tothe benzene ring (the alkylbenzene derivative having a quaternary carbondirectly bonded to the benzene ring being mainly adsorbed on the basalsurface of the carbon, and the unsaturated cyclic carbonate being mainlyadsorbed on the edge surface of the carbon). This causes the film to beformed more efficiently, thereby preventing the cycloalkylbenzenederivative from making contact with the negative electrode and frombeing decomposed.

The cell X1, in which no additives were added, the cells X4 and X5, inwhich only the alkylbenzene derivative having a quaternary carbondirectly bonded to the benzene ring was added, and the cell X8, in whichonly the unsaturated cyclic carbonate was added, had abnormality such assmoking. This is presumably because sufficient prevention of overchargeis impossible without additives, and because the alkylbenzene derivativehaving a quaternary carbon directly bonded to the benzene ring and theunsaturated cyclic carbonate have virtually no effectiveness inpreventing overcharge.

The cells X2 and X3, in which only the cycloalkylbenzene derivative wasadded, the cell X6, in which only the cumene (alkylbenzene derivativehaving a tertiary carbon directly bonded to the benzene ring) was added,and the cell X7, in which only the trimellitic ester (benzene derivativein which the quaternary carbon directly bonded to the benzene ring is acarboxyl carbon) was added, had high-temperature cycle characteristicsof 54%, which is lower than 62% for the cell X1 without additives addedtherein. This is presumably because the cycloalkylbenzene derivative,the alkylbenzene derivative having a tertiary carbon directly bonded tothe benzene ring, and the benzene derivative in which the quaternarycarbon directly bonded to the benzene ring is a carboxyl carbon aredecomposed under a high temperature to form a highly resistive film onthe negative electrode.

The results of the cells A1 to A4 and A8 to A11 show that when thecycloalkylbenzene derivative is contained in the non-aqueous solvent ata ratio of 0.5 to 5 parts by mass per 100 parts by mass of thenon-aqueous solvent excellent cell characteristics are obtained;specifically, the high temperature cycle characteristics is 58% orhigher in the cells without the unsaturated cyclic carbonate addedtherein and the high temperature cycle characteristics is 80% or higherin the cells with the unsaturated cyclic carbonate added therein. Thus,the amount of the cycloalkylbenzene derivative added is preferablycontrolled within the above-described range.

The results of the cells A1, A5 to A8, and A12 to A14 show that when thealkylbenzene derivative having a quaternary carbon directly bonded tothe benzene ring is contained in the non-aqueous solvent at a ratio of0.5 to 10 parts by mass per 100 parts by mass of the non-aqueous solventexcellent cell characteristics are obtained; specifically, the hightemperature cycle characteristics is 58% or higher in the cells withoutthe unsaturated cyclic carbonate added therein and the high temperaturecycle characteristics is 80% or higher in the cells with the unsaturatedcyclic carbonate added therein. Thus, the added amount of thealkylbenzene derivative having a quaternary carbon directly bonded tothe benzene ring is preferably controlled within the above-describedrange.

While in the above Examples cylindrical cells were prepared, the presentinvention is not limited to such a shape; the invention can also beapplied to cells of various shapes such as coin-shaped cells,square-shaped cells, and laminate cells. It is also possible to applythe present invention to gel non-aqueous electrolyte secondary cellsusing a polymer electrolyte.

While in the above Examples cells having the current-cutting device wereprepared, the present invention can also be applied to cells notprovided with the current-cutting device. In this case, thecycloalkylbenzene derivative is decomposed at the time of overcharge toform a highly resistive film and generates a hydrogen gas to increaseinternal resistance, thereby preventing overcharge.

While in the above Examples cells using cyclohexylbenzene as thecycloalkylbenzene derivative and using tert-amylbenzene as thealkylbenzene derivative having a quaternary carbon directly bonded tothe benzene ring were used, the present invention is not limited to theabove compounds. For instance, a cell using cyclopentylbenzene as thecycloalkylbenzene derivative and using tert-butylbenzene as thealkylbenzene derivative having a quaternary carbon directly bonded tothe benzene ring can also realize good high temperature cyclecharacteristics and safety such that no abnormality occurs at the timeof overcharge. Further, while vinylene carbonate was used as theunsaturated cyclic carbonate, the present invention is not limited tothis compound. For instance, a cell using methyl vinylene carbonate orthe like also can realize good high temperature cycle characteristicsand safety such that no abnormality occurs at the time of overcharge.

INDUSTRIAL APPLICABILITY

As has been described above, the non-aqueous electrolyte secondary cellaccording to the present invention is excellent in high temperaturecycle characteristics and in safety such that no abnormality occurs atthe time of overcharge. Thus, the industrial applicability of thepresent invention is considerable.

1. A non-aqueous electrolyte secondary cell comprising: a positiveelectrode for reversibly intercalating-deintercalating lithium ions; anegative electrode for reversibly intercalating-deintercalating lithiumions; and a non-aqueous electrolyte having a non-aqueous solvent and anelectrolyte salt, wherein, the non-aqueous solvent includes acycloalkylbenzene derivative, tert-amylbenzene, and an unsaturatedcyclic carbonate derivative.
 2. The non-aqueous electrolyte secondarycell according to claim 1, wherein the unsaturated cyclic carbonatederivative is vinylene carbonate contained in the non-aqueous solvent ata ratio of 0.5 to 10 parts by mass per 100 parts by mass of thenon-aqueous solvent.
 3. The non-aqueous electrolyte secondary cellaccording to claim 1, wherein: the cycloalkylbenzene derivative iscontained in the non-aqueous solvent at a ratio of 0.5 to 5 parts bymass per 100 parts by mass of the non-aqueous solvent; and thetert-amylbenzene is contained in the non-aqueous solvent at a ratio of0.5 to 10 parts by mass per 100 parts by mass of the non-aqueoussolvent.
 4. The non-aqueous electrolyte secondary cell according toclaim 2, wherein: the cycloalkylbenzene derivative is contained in thenon-aqueous solvent at a ratio of 0.5 to 5 parts by mass per 100 partsby mass of the non-aqueous solvent; and the tert-amylbenzene iscontained in the non-aqueous solvent at a ratio of 0.5 to 10 parts bymass per 100 parts by mass of the non-aqueous solvent.